The use of polymer coatings to protect surfaces and enhance the aesthetic and functional properties of materials is well known. These polymer coatings are typically applied to surfaces as liquid systems using techniques such as rolling, brushing, sprinkling, casting and pneumatic or electrostatic spraying.
There is a need to control the rheological profile of these liquid systems such that the coating can be applied to a substrate by the method of choice without problems and then flow evenly over the substrate, allowing surface unevenness introduced by the application step, as well as unevenness from the underlying substrate, to be leveled out as much as possible. Concomitantly the applied liquid film should not be allowed to sag or form tears on non-horizontal substrates. Such sagging can be particularly pronounced when thicker film coatings are applied and where the substrate has features such as at edges, holes and character lines which can promote localized build-up of the film.
As a consequence of these requirements the use of rheology modification agents in coating compositions to create a pseudoplastic, thixotropic rheology profile, to limit or prevent sagging defects is widespread; it is particularly important in the automotive coating market—and more particularly for clear coatings used in that industry—where obtaining extremely well-leveled films without sagging complications is of the highest importance.
Following the step of applying a coating composition to a substrate many coatings are subjected to curing and/or drying processes; it is equally important to control the rheological profile of the coating at these later stages. For instance when a fluid top coat layer applied to a substrate is shrinking due in part to the release of volatile compounds, the profile of the underlying substrate can be transferred to the coat layer. This process is known as “telegraphing” and is independent of the roughness originally introduced in the spraying step. For applications which use a heating step following the application and an optional flash-off period at low temperature, the spraying roughness may have been allowed to level to a great extent, but solvent evaporation may introduce new roughness in these late stages of drying/curing, thereby disturbing an originally well-leveled surface. The more shrinkage of the top coat, the more pronounced this transfer of the substrate roughness may be. This telegraphed roughness which develops during the drying and/or curing process can still be leveled out, provided that the coating film still has enough mobility (“fluidity”) left after (partial) development of the telegraphed roughness and before it finally crosslinks.
Other physical phenomena related, for example, to layer thickness variations on non-perfect substrates and to the impact of developed surface tension gradients can also contribute to late stage introduction of new surface roughness and also require late stage fluidity to be leveled out again.
There is a need for a rheology control agent for coating applications with a curing cycle effected at temperatures of more than 60° C., that allows for a good balance between effective sagging reduction of a coating formulation, that extends far enough in the heating cycle, and on the other hand provides not only for sufficient leveling of the spraying roughness originally introduced, but also for leveling of substrate roughness developing only in the latest stages of the drying/curing cycle.
Presently available rheology control agents tend to suffer from deviations from this ideal balance to one side or another by failing to compensate for the conflicting rheological requirements throughout the application, curing and drying stages.
Rheology control agents—such as organoclays and fumed silica—have been employed to limit the sagging of clear coat formulations in which a high temperature cure process is used. Such rheology control agents are described in C. H. Hare, “Thixotropes and additives for modifying paint flow”, J. Prot. Coat Linings, No. 79 (April 2001). The rheology control agents limit sagging by building up a high structure viscosity after application. If this structure viscosity is lost too rapidly in the curing cycle, sagging may still occur in the later stages of the curing because of the low viscosity of the hot coating film. Therefore, these rheology control agents act to maintain a high viscosity during the curing cycle. However, such high-temperature active rheology control agents limit the potential of the coating to allow for the leveling of telegraphed roughness developing only in the late stages of drying/curing, with a negative effect on appearance.